It at be happen due to many reasons but the appropriate reason is that it may due to your foot wear especially with long heals. And due to wrong steeping in the ground. We can eliminate this type of issues with a proper treatment like going for jogging in the morning time and some leg exercises.

When you are born your mother most likely played with your feet causing them to turn outwards. If you jump roped as a child landing on the feet to much had made them turn over time. The more you jump the more they turn. Till they are Backwards D: